Degaussing circuit for wide-range ac

Abstract

An automatic degaussing circuit provides a video display apparatus with a degaussing capability over a wide range of ac mains sources. An ac mains source is coupled to a degaussing coil by a series interconnection of first and second temperature-sensitive resistances. The temperature-sensitive resistances have oppositely-tending temperature coefficients, so that a total dc impedance of the automatic degaussing circuit initially decreases after power is applied to the circuit.

What is claimed is: 
     
       1. A degaussing circuit for a cathode ray tube, comprising: a source of alternating voltage potential;   a switch means coupled to said source;   a first temperature-sensitive device coupled to said switch means;   a second temperature-sensitive device coupled to said first device; and   a degaussing coil coupled to said second temperature-sensitive device;   wherein a degaussing current flowing through said first and second temperature-sensitive devices is equal to a degaussing current flowing through said degaussing coil.   
     
     
       2. The degaussing circuit of claim 1, wherein a maximum peak-to-peak value of the degaussing current flowing through said degaussing coil does not occur during a first cycle of said degaussing current. 
     
     
       3. The degaussing circuit of claim 1, wherein said switch means comprises a relay. 
     
     
       4. The degaussing circuit of claim 1, wherein said first and second temperature-sensitive devices comprise resistances. 
     
     
       5. The degaussing circuit of claim 1, wherein said first temperature-sensitive device has a positive temperature coefficient and said second temperature-sensitive device has a negative temperature coefficient. 
     
     
       6. The degaussing circuit of claim 5, wherein: said first temperature-sensitive device has a first terminal coupled to said switch means and a second terminal; and,   said second temperature-sensitive device has a first terminal coupled to said second terminal of said first temperature-sensitive device and a second terminal coupled to said degaussing coil.   
     
     
       7. The degaussing circuit of claim 1, wherein said alternating voltage potential has a root-mean-square value in the range between approximately 90 volts rms and approximately 270 volts rms. 
     
     
       8. The degaussing circuit of claim 1, wherein said alternating voltage potential has a frequency in the range between approximately 50 Hz and approximately 60 Hz. 
     
     
       9. The degaussing circuit of claim 1, wherein said first and second temperature-sensitive devices are not in thermal contact. 
     
     
       10. The degaussing circuit of claim 1, wherein said degaussing circuit does not generate a significant residual magnetization.